Hitherto, there have been known methods for synthesizing β-glycoside compounds from dihydrofuran compounds or the like, and such methods are disclosed in several documents (see, for example, Non-Patent Document 1). In the method disclosed in Document 1, a mixture of an acetylglycoside compound of α-form and the same compound of β-form serving as a raw material is caused to react with a phosphine ligand and a palladium reagent (catalyst), to thereby synthesize a corresponding β-glycoside compound. Document 1 also discloses that an optically active Trost ligand must be used in order to selectively synthesize a β-glycoside compound without forming an α-glycoside compound, and that use of optically inactive diphenylphosphinoferrocene as a ligand results in loss of selectivity, merely yielding a glycoside compound in the α and β mixed form.
In one known case, a specific cyclopentene compound is coupled with ethyl nitroacetate in the presence of tetrakis(triphenylphosphine)palladium. However, α or β selectivity of the product is not disclosed (see, for example, Non-Patent Document 2).
It has been reported that a specific cyclopentene compound (α-form and β-form) serving as a raw material is coupled with imidazole in the presence of tetrakis(triphenylphosphine)palladium, to thereby yield a product (α-form and β-form) with no selectivity (see, for example, Non-Patent Document 3).
Also known is a case in which a specific dihydropyran compound is coupled with a nitromalonic acid ester in the presence of tetrakis(triphenylphosphine)palladium. In this case, reactivity of the α-form raw material is completely the same as that of the β-form raw material (see Non-Patent Document 4).
Yet also known is a case in which a specific cyclopentene compound is coupled with a nucleophile in the presence of tetrakis(triphenylphosphine)palladium. When the nucleophile is adenine, a β-glycoside compound is exceptionally formed. However, when the nucleophile is thymine, no specific selectivity has been reported. Furthermore, it is suggested that, depending on the type of the functional group(s) of the cyclopentene compound, a β-glycoside compound could be selectively yielded only when an optically active Trost ligand is used (see, for example, Non-Patent Document 5).
As described above, those skilled in the art know that difficulty is encountered in selectively yielding a β-glycoside compound through coupling a cyclic alkene compound in the α and β mixed form serving as a raw material in the presence of a transition metal catalyst, unless an expensive optically active ligand is employed.
Meanwhile, β-glycosylation of a compound having a dihydrofuran ring is known to proceed in the presence of a Lewis acid (see, for example, Patent Document 1).